On the first 5 exams of his physics class, Omar got an average score of 88. What does he need on the next exam to have an overall average of 90?
Solution: Let his score on the next exam be $x$ The sum of all of his scores is then $5 \cdot 88 + x$ The same sum must also be equal to $6 \cdot 90$ Solve: $x = 6 \cdot 90 - 5 \cdot 88 = 100$.